In optical communication systems that use polarization multiplexing (POLMUX), two optical signals are polarized orthogonally with respect to each other before being transmitted in an optical channel at a certain wavelength. This doubles the data rate of the channel. To realize a high-speed transmission over 100 Gbps, both POLMUX and dense-wavelength-division multiplexing (DWDM) are needed.
However, the constantly alternating bit sequences of the POLMUX signal in DWDM systems cause the resulting polarization to change over time. This depolarization effect results in cross-polarization modulation (XPolM), which is essentially noise. The XPolM impairment is induced by inter-channel fiber nonlinearity particularly for DWDM optical communication systems. Due to the XPolM, the dual-polarized (DP) signals, which originally were polarized orthogonally, are no longer so, and cannot be separated at a receiver by a polarization splitter.
XPolM causes significant performance degradation in bit error rate or in quality factor for DP coherent optical systems. The induced polarization crosstalk has a low-pass response because of chromatic dispersion, resulting in a short coherence time of tens of symbols. Such a fast time-varying polarization crosstalk is hard to be compensated at receivers for current optical communication systems.
Most conventional techniques focus on compensating the time-varying XPolM effect at the receiver, through the use of a crosstalk cancellation with adaptive channel estimation. However, the performance improvement is limited by the channel estimation accuracy, which is severely degraded by rapid time-varying channels, particularly due to the XPolM. In addition, nonlinear compensation methods at the receiver have a high-computational complexity to implement in circuits.
Some nonlinear rotations such as self-phase modulation (SPM) are well treated by differential coding schemes like dual-polarized differential quadrature phase-shift keying (DP-DQPSK) even without accurate channel estimations. However, DP-DQPSK suffers from severe performance degradations in the presence of polarization crosstalk due to XPolM and polarization mode dispersion (PMD).